Mechanical performance in hydrogels ranges from very soft and brittle gels to extremely tough and stiff gels, all of which may be widely applied. Conventional hydrogels, such as highly crosslinked polyvinyl alcohols, have been particularly successful in low-load bearing biomedical applications including drug delivery, wound dressings, and injectable fillers, due to their high water content and biocompatibility. The low-load bearing limits of conventional hydrogels are a product of the intrinsic heterogeneity and brittle primary network. During synthesis, the initial differences in reactivity between monomers and crosslinkers create densely crosslinked microgels, which later assemble into macrogels. This inhomogeneous gel formation causes regions of high strain on the polymer system exacerbated during swelling. Thus, the system has few mechanisms available to distribute load effectively, leading to network failure.
Great interest exists for expanding the scope of hydrogel applications in more mechanically demanding environments by bridging the gap between these conventional, weaker hydrogel networks and tough polymeric elastomers. Double network hydrogels (DN) can absorb large amounts of energy and attain a very high modulus but with significant disadvantages. These DN hydrogels compensate for the brittleness and low modulus of conventional single network hydrogels by reinforcing them with a second or third interpenetrating network, introduced at reduced osmotic stress relative to the primary network. In most configurations, a high initial stress state in the primary network produces an enhanced modulus while the secondary and tertiary networks provide enhanced structure and some additional absorption of energy even after the high modulus primary network fails. This method of toughening through additional networks has even been incorporated into non-hydrated elastomers with good success, but is not compatible with fatigue resistance as the primary network is no longer intact for subsequent loading cycles. Like single network hydrogels with low fatigue resistance, the failure mode of the primary network is also correlated to its heterogeneity, leading fragmentation into microgels. These microgels then act as sliding crosslinkers for the secondary network, giving additional energy absorption capabilities, but no longer providing access to the high strain and stress capabilities exhibited under initial loading.
Many advanced applications of hydrogels need to undergo very high levels of cyclic loading. The permanent mechanical fatigue present in many DN and TN gels is incompatible with such applications. Healable primary networks may remedy this issue if incorporated into a DN hydrogel; however, rates of recovery are usually very slow (>12 hours), making them impractical for applications requiring high frequency cyclic loading. There is a long-felt, but unmet need for hydrogel systems that possesses the significant modulus and fatigue resistance (toughness) of such DN and TN systems, while also being able to undergo high levels of cyclic loading without experiencing permanent mechanical fatigue.